Field of the Invention
The present invention relates to a blade for a fan of a turbomachine, notably of unducted fan type, a corresponding fan and a corresponding turbomachine.
Although the present invention is particularly suited to turbomachines with unducted fan (also described as “open rotor”), its use is not however limited to such an application.
Description of the Related Art
As is known, a turbomachine with unducted fan can comprise two coaxial and contra-rotating outer fans, respectively upstream (front) and downstream (rear), which are each driven in rotation by a turbine and which extend, substantially radially, outside the nacelle of the turbomachine. Each fan usually comprises a hub that is concentric to the longitudinal axis of the turbomachine, with blades fastened to it.
However, the aerodynamic interaction between the upstream and downstream contra-rotating fans of such a turbomachine with unducted fan generates high operating acoustic levels. In practice, the rotation of the blades of the upstream and downstream contra-rotating fans provokes the formation of:                wakes along the span of the blades, downstream thereof;        marginal vortices at the free end of the blades; and        turbulences at the base of the blades.        
These aerodynamic disturbances downstream of the upstream fan are the source of aerodynamic interaction noise when they strike the downstream fan or pass in proximity thereto.
In particular, during phases of low-speed operation of a turbomachine with unducted fan (such as, when it is mounted on an aircraft, when taxiing, taking off, landing and on approach), the dominant contribution of the radiated noise originates from the lines of interaction associated with the downstream fan which is operating in the stream from the upstream fan, by passing through the vortex sheets made up of wakes and marginal vortices formed by the blades of the upstream fan (also called upstream blades). When a marginal vortex from upstream blades interacts with the blades of the downstream fan (otherwise called downstream blades), the downstream blade-marginal vortex interaction dominates the radiated acoustic spectrum for most of the directivities.
Also, to reduce the undesirable sound emissions from such turbomachines and thus satisfy the acoustic certification criteria imposed by the aeronautical authorities, it is necessary to reduce the radiated noise at low speed by reducing the downstream blade-marginal vortex interaction.
Currently, the most widely-used known solution—called clipping—consists in reducing the diameter of the downstream fan, so as to make the marginal vortices generated by the upstream blades pass outside the downstream blades to limit the interaction of the latter with the marginal vortices. This generally entails increasing the chord of the downstream blades to maintain the desired traction and the torque ratio between the upstream and downstream fans. Such a solution can be pushed to the limit by very strongly loading the end of the upstream blades, so as to unload the rest of each of the upstream blades to reduce the impact of the wake from the upstream fan on the downstream fan, which is also the source of undesirable interaction noise.
However, such a solution proves acceptable only for an isolated and incidence-free configuration of the turbomachine (that is to say, with no external element linked to it). In the presence of elements (strut, fuselage) or of incidence, the contraction and the axisymmetry of the air flow behind the upstream fan are modified, so that the clipping produced no longer prevents the interaction of the downstream blades and of the marginal vortices generated by the upstream blades. A significant reduction of the height of the downstream blades (corresponding to a significant clipping) entails increasing the chord associated with the downstream blades so as to retain the load, which degrades the efficiency of the associated turbomachine and is not therefore satisfactory.